The invention relates generally to the field of alloy castings, and more particularly to liquid cooled alloy castings and methods of making the same.
Generally, superalloy castings are formed by pouring a molten metal in a mold at a high temperature greater than about 700° C. in a mold. The mold is typically placed in a liquid metal bath to cool the mold and facilitate the solidification of the melt. It is desirable to have such metals in the liquid metal bath that have relatively low density, low melting point (less than 700° C.) and low cost. Metals that are typically employed in the metal bath include lithium (186° C.), sodium (98° C.), magnesium (650° C.), aluminum (660° C.), potassium (63° C.), zinc (419° C.), indium (156° C.), tin (232° C.), antimony (630° C.), cesium (28° C.), tellurium (450° C.), and the like. Metals such as lithium, sodium, potassium and cesium have low melting temperatures, but they are very flammable, thereby causing safety concerns. On the other hand, metals such as zinc and antimony have low vapor pressures due to which these metals evaporate and pose a risk of contaminating the casting alloy. Some of the other metals, such as mercury, are toxic in nature. Aluminum and tin are generally preferred metals for liquid metal bath. Aluminum has a higher melting point than tin. As will be appreciated, heat transfer between a casting and coolant (liquid metal bath) is a function of temperature differences; therefore, liquid tin due to its lower melting point is typically preferred over liquid aluminum in removing heat from the castings.
Due to the very high temperatures involved in the casting process, the molds are prone to cracks. For example, a mismatch between the coefficients of thermal expansion of the mold and the superalloy inside the mold may result in cracks in the mold. Once the cracks have developed, the liquid tin may seep into the mold and contaminate the casting. The liquid tin attacks the surface of the casting causing pits on the surface of the casting. As will be appreciated, a pit is a defect extending from the surface towards the core of the casting. Pitting is caused due to reaction of tin with components of the superalloy that forms tin compounds.
A plausible solution to avoid pitting in such castings is to make a mold that does not crack, thus preventing liquid metal from contacting the casting surface. However, it has not been possible to make such a mold. Additionally, such a strong mold may crack the casting due to thermal expansion mismatch between the casting and the mold.
Accordingly, there is a need to provide a liquid metal attack resistant layer for castings to improve the surface finish of the castings by reducing or eliminating pitting in the castings.